Cable television systems have made possible the transmission of many channels of television programs to the homes of subscribers. Instead of being limited to the number of VHF and UHF channels that can be transmitted and received in a given area, the number of channels in cable systems is limited only by the transmission characteristics of the cable itself and the ability to compress the information in television programs into narrower-bandwidth channels. The advent of fiber-optic transmission systems for use in cable television networks has also vastly increased the number of channels available.
Such increase in channels has also given rise to proposals for interactive television systems wherein a subscriber can transmit information or requests back into the system, which information or requests may subsequently affect programs or information directed to such subscriber. There are a wide variety of applications for interactive television systems, such as video games, video catalog shopping, teaching systems, movies on demand and audio programs. Each application can be tailored for an individual subscriber, for example a subscriber may be able to select the language of the soundtrack in a movie. However, such systems typically require the ability (i) to control specific programs or information sent to each subscriber and (ii) to receive input messages or requests from the subscriber.
A headend-to-subscriber channel does not need the full bandwidth of the usual television cable channel if compression can be used. For example, a number of compressed and digitized television signals can be transmitted over a single conventional 6 Mhz cable channel. Other program information, such as high-fidelity audio, still video pictures or text can also be sent in compressed form. By using a large number of conventional channels for ITV purposes, such as is now possible with optical fiber distribution systems, together with compression, it is conceivable that hundreds of "virtual" channels could be made available. Such virtual channels can also be used to transmit control information to subscriber locations.
In many cable television systems, a special converter is used at the subscriber location to allow the subscriber to select among the various available channels, and possibly to "unscramble" premium channels for which extra fees are payable. Such converters usually provide an output on one of the channels that a standard television receiver can receive, such as Channel 3. More recently, television receivers and video recorders have been made available that are "cable ready," that is, with the ability to receive and select among all the channels transmitted over the cable system, but without the ability to unscramble premium channels. A converter is still needed for the premium channels.
Many cable-ready television receivers have additional features, such as picture-within-picture and remote-control tuning, that cannot be used conveniently with cable converters that provide an output over a single channel. Similarly, video recorders capable of being programmed to record selected channels at selected times are defeated by such a converter. Accordingly, it is desired to provide a converter for use in interactive television systems that is "transparent" to unrestricted channels on the cable system and that permits use of cable-ready features on television receivers and video recorders.